The present invention relates to a power steering apparatus which applies a steering assist force to a steering mechanism of a motor vehicle by a hydraulic pressure generated by a pump driven by an electric motor.
Power steering apparatuses are known which assist operation of a steering wheel of a motor vehicle by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the resolutions per minute (rpm) of the electric motor.
Drive control of the electric motor is performed, for example, on the basis of the steering angle of the steering wheel. That is, the steering angle is determined on the basis of an output of a steering angle senior provided in association with the steering wheel, and the driving of the electric motor is controlled on the basis of the steering angle. More specifically, if the steering angle of the steering wheel is within a motor stop range defined around a steering angle midpoint, steering assist is considered to be unnecessary, so that the electric motor is stopped. On the other hand, if the steering angle of the steering wheel is out of the motor stop range, the electric motor is driven to generate a steering assist force.
Detection of the steering angle midpoint is achieved, for example, by sampling steering angle data outputted from the steering angle sensor and regarding the most frequent steering angle data as corresponding to the steering angle midpoint.
However, the power steering apparatus described above performs the drive control of the electric motor with the use of the steering angle sensor, so that the drive control cannot be performed on the basis of the steering torque. Therefore, the sense of steering control is not satisfactory to the driver.
Where a motor vehicle travels along a straight road inclined transversely of the vehicle, for example, a torque should be applied to the steering wheel for stabilization of the vehicle even though the steering angle of the steering wheel is small enough to be within the motor stop range. In such a case, the aforesaid power steering apparatus does not provide the steering assist even under application of the steering torque. Hence, the sense of steering control for the driver is deteriorated.
A torque sensor may be used instead of the steering angle sensor to perform the drive control in accordance with the steering torque. However, the use of the torque sensor is not preferable, because the torque sensor is less reliable than the steering angle sensor.
It is an object of the present invention to provide a power steering apparatus which can ensure an improved sense of steering control for the driver.
It is another object of the present invention to provide a power steering apparatus which is capable of controlling driving of an electric motor in accordance with a steering torque to properly provide steering assist without the need for a torque sensor.
It is further another object of the present invention to provide a power steering apparatus which can ensure enhancement of energy saving.
A power steering apparatus of the present invention, which is adapted to generate a steering assist force by a hydraulic pressure generated by a pump driven by an electric motor, comprises: steering angular speed detecting means for detecting a steering angular speed; electric current detecting means for detecting the value of motor current flowing through the electric motor; and stop controlling means for stopping the electric motor in response to duration of a predetermined time period during which the steering angular speed detected by the steering angular speed detecting means is kept at a value that does not exceed a predetermined stop threshold and the motor electric current value detected by the electric current detecting means is kept within a predetermined motor stop range.
The motor current value of the electric motor varies depending upon a load applied to the motor, i.e., a steering torque. Therefore, when a relatively large steering torque is applied to a steering mechanism, the electric current value of the electric motor is relatively high even if the steering angle has a value close to a steering angle midpoint. In such a case, the electric motor is not stopped in accordance with the present invention. Since the electric motor can be driven when a steering torque is applied, the driving of the electric motor can be controlled according to the steering torque even without the torque sensor. Thus, the sense of steering control for the driver is improved.
In accordance with one embodiment of the present invention, the power steering apparatus further comprises: non-load electric current value calculating means for determining, on the basis of the motor electric current value detected by the electric current detecting means, a non-load electric current value which corresponds to a motor electric current value observed when the electric motor is in a non-load state; and motor stop range determining means for determining the motor stop range on the basis of the non-load electric current value determined by the non-load electric current value calculating means.
When the steering torque is zero, the electric motor is in the non-load state. Therefore, it is considered that, when the motor electric current value is within a predetermined range around the non-load electric current value, the steering torque is virtually zero, so that no steering assist is required. By determining the motor stop range on the basis of the non-load electric current value, stop control of the motor can properly be performed.
The motor stop range determining means is preferably adapted to determine, as the motor stop range, a range defined between the non-load electric current value and a value obtained by adding a predetermined electric current threshold to the non-load electric current value.
The electric current threshold is preferably determined on the basis of a torque range where a motor vehicle equipped with the power steering apparatus requires no steering assist.
Another power steering apparatus of the present invention, which is adapted to generate a steering assist force by a hydraulic pressure generated by a pump driven by an electric motor, comprises: steering angle detecting means for detecting a steering angle with respect to a steering angle midpoint; actuation control means for actuating the electric motor on condition that a change amount of the steering angle detected by the steering angle detecting means exceeds a predetermined actuation threshold in a motor stop state; and actuation threshold determining means for determining the actuation threshold in accordance with a value of the steering angle detected by the steering angle detecting means when the electric motor is stopped.
Where the steering angle has a value close to the steering angle midpoint, the steering angle changes by a relatively large change amount to get out of a play angle range of the steering wheel. That is, a large amount of the change in the steering angle is required until the steering assist becomes necessary. On the contrary, where the steering angle is relatively large, a great steering assist force is required immediately after the steering operation is started.
In the present invention, the actuation threshold is determined in accordance with the steering angle detected when the electric motor is stopped, and the electric motor is actuated on condition that the change amount of the steering angle exceeds the actuation threshold. Therefore, the actuation threshold can be set higher when the steering angle at the stop of the motor is close to the steering angle midpoint, and set lower when the steering angle (absolute value thereof) at the motor stop is relatively large. Thus, the electric motor is not actuated needlessly when the steering angle is close to the steering angle midpoint, and a great steering assist force can immediately be generated when the steering angle is large. Therefore, both the enhancement of the energy saving and the improvement of the sense of steering for the driver can be achieved.
The actuation control means may include actuation steering angle calculating means for determining, on the basis of the value of the steering angle detected by the steering angle detecting means and the actuation threshold determined by the actuation threshold determining means, an actuation steering angle which corresponds to a steering angle at which the electric motor in an off state is to be actuated; and means for actuating the electric motor, when the electric motor is off, on condition that the value of the steering angle detected by the steering angle detecting means reaches the actuation steering angle determined by the actuation steering angle calculating means.
Further, the actuation threshold determining means is preferably adapted to set the actuation threshold lower as the value of the steering angle detected by the steering angle detecting means at the motor stop increases.
In accordance with another embodiment of the present invention, the power steering apparatus further comprises vehicle speed detecting means for detecting a vehicle speed, and the actuation threshold determining means is adapted to set the actuation threshold higher as the vehicle speed detected by the vehicle speed detecting means increases.
Thus, immediate motor actuation is ensured at low speed traveling, and motor actuation sensitivity is reduced at high speed traveling. Therefore, needless actuation of the electric motor is prevented at a high traveling speed at which the steering assist is less necessary, while the steering assist is immediately started at a low traveling speed. This enhances the energy saving as well as the sense of steering control for the driver.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the embodiment with reference to the attached drawings.